The present invention relates to a dielectric material in the shape of a sheet which may be employed in an electric wire, a cable, a printed circuit board and in other electric signal transmission devices.
As a dielectric for electronic machines or devices, an open-cell type oriented porous, expanded polytetrafluoroethylene (PTFE) is known. This material has a relatively low inductivity and is stable both physically and chemically. This type of material can be produced by the method disclosed in the specification of U.S. Pat. No. 3,953,566. This method enables production of a material having a porosity of 90% or more. However, the polymeric material produced by the conventional method is highly compressible and easily collapses and thereby becomes decreased in porosity, resulting in an increase in permittivity. As a result, it is difficult to handle such product.
Accordingly, it is an object of the present invention to provide a sheet-shaped dielectric which is not readily collapsed and has a lowered permittivity.